This invention relates to the recovery of metals from waste or scrap materials and in particular to a process in which free metals are obtained by reduction of chemically combined forms of the metals under mild, readily controlled conditions.
The reducing capability of the alkali metals, lithium, sodium, potassium, rubidium and cesium, is well known. The oxidation potentials of these metals range from +2.71 volts for sodium to +3.05 volts for lithium, the oxidation potential being a measure of the reducing strength of the metal. These high values place the alkali metal at the top of the electromotive force series, indicating that, in theory, they will reduce virtually any oxidizing agent no matter how weak.
As a practical matter the full reducing capability of the alkali metals has remained largely unrealized. For example, sodium has been used extensively in the form of its solution in liquid ammonia for the production of metals from their salts, however, its reducing properties are appreciably diminished in this form, in comparison with the pure metal. Reduction processes which have been developed heretofore employing pure alkali metals have proven to be generally unsatisfactory in that they are operable on a relatively small scale, require specialized equipment, and often involve rather extreme operating conditions, such as complete exclusion of air and water.
Presently, considerable amounts of precious and other useful metals are disposed of each year because of the difficulty of recovering the metal economically by currently available technology. For example, exposed photographic film containing silver salts, and mine tailings containing appreciable quantities of metal ores are simply discarded because recovery by presently available methods is generally unprofitable.
The desirability of a commercially practicable method employing alkali metals for the recovery of a wide variety of less reactive metals by chemical reduction has led to the development of the present invention.